In propeller assemblies for aircraft power plants, aerodynamic and engine induced forces place high bending moments on the propeller blades. While the blades are ordinarily are ordinarily more or less rigid structurally, they deflect under operating loads and are thus subjected to large operational stresses, which are generally vibratory in character. Various proposals have been made to minimize or eliminate bending moments in propeller blades. The proposals have included: (a) an articulating connection between each propeller blade and the propeller hub; (b) an elastic mounting of the entire propeller on its drive shaft; and (c) an elastic connection between each propeller blade and the propeller hub.
The last mentioned proposal has recently gained considerable favor, particularly in the helicopter industry. While older proposals for such an elastic or resilient connection merely utilize a body of elastomer, as shown, for example, in Le Compte et al. U.S. Pat. No. 2,631,680, more recent designs have utilized high-capacity, laminated elastomeric bearings. Such bearings consist of alternating layers of an elastomer, such as rubber, and a rigid material, such as metal, bonded together to produce a bearing that can carry extremely high compression loads normal to the layers, yet can accommodate a high degree of pivotal movement along the layers through shearing of the elastomer layers. Examples of mounting arrangements for helicopter blades which utilize laminated elastomeric bearings can be seen in: FIG. 5 of Reed U.S. Pat. No. 3,484,172; Gorndt et al. U.S. Pat. No. 3,106,965; Schmidt U.S. Pat. No. 3,292,712; Gorndt U.S. Pat. No. 3,700,352; Rybicki et al. U.S. Pat. No. 3,764,230; and Gorndt et al. U.S. Pat. No. 3,862,812. (The Gorndt and Schmidt patents are all assigned to the assignee of the present invention.) As utilized in helicopter rotors, laminated elastomeric bearings accommodate the various blade motions that are encountered during flight, such as lead-lag movement, flapping, and blade pitch variations. To permit the necessary pivotal movements of the blades, the bearings must be relatively large.
In fixed wing aircraft, it is customary to mount the propeller blades rigidly on the propeller hub or hubs, except for the use of frictionless bearings to permit changes in blade pitch. To keep vibratory operational stresses within acceptable limits, the blades are configured so as to detune them from the exciting frequencies. Since the exciting frequencies vary with different aircraft engines, propeller blades must often be redesigned for use with different engines. Such redesigning has been accepted in the industry at least partly because the Federal Aviation Administration certifies aircraft engines and propellers together as complete assemblies, rather than as separate items. While elastomeric bearings offer the opportunity to eliminate or at least reduce blade redesigns, the propeller hubs of fixed-wing aircraft, particularly light planes having variable pitch propellers, are not large enough, nor can they be conveniently enlarged, to accommodate large laminated elastomeric bearings. Accordingly, a laminated elastomeric bearing cannot be used in such an installation to perform both a stress-relief function and a variable pitch accommodation function. If elastomeric bearings are to be used at all in the propellers of fixed-wing aircraft, the bearings must be kept small in size by using them in combination with roller or ball bearings, in the manner illustrated in the Le Compte and Reed patents noted above. The ball bearings or roller bearings will accommodate the large rotational movements associated with changes in pitch, while the elastomeric bearings will accommodate the smaller movements resulting from blade vibration.